1. Field of the Invention
The invention relates to the field of high-frequency DC-DC converters in which electrical power at a first voltage is transferred from a power source to a load at a second voltage by high-frequency switching of current through circuits containing inductive and other resonant components. In particular, the invention relates to a family of multiphase converters in which two or more converter circuit slices are used to transfer electrical energy in parallel by switching current through the circuits sequentially in packets of energy.
2. Description of the Related Art
A DC-to-DC converter is a circuit which converts a direct current voltage at one level to a direct current voltage at another level. One of the primary design goals in such DC-to-DC converters is to increase the amount of power transferred through the converter. Common prior art DC-to-DC converter topologies include the buck or forward converter, the buck-boost or flyback converter, and the boost converter, which transfer energy from the input to the output during part of the switching cycle. In these circuits a dead time was created during the energy transfer (discontinuous energy transfer) which resulted in the need for larger output filters. Subsequent designs, see for example U.S. Pat. No. 4,734,839 (Barthold), used combinations of topologies to achieve a continuous transfer of energy from the input to the output of the converter, which allowed a significant reduced size of output filter.
Increasing the switching frequency is also known to enable increased power transfer through the converter. This can also result in a reduction in the size of the output filter and, in the case of converters using galvanic isolation transformers, a reduced size of the isolation transformer. However, as frequency increases, so switching losses in semiconductor switches start to increase significantly, due to the finite switching speed or the time required for the current in the semiconductor device to start and stop flowing.
In order to overcome the problems inherent in using higher switching speeds, resonant and quasi-resonant DC-to-DC converters were developed which permitted zero-current (ZCS) and zero-voltage (ZVS) switching. In the case of quasi-resonant converters, the current or voltage is shaped to become half-sinusoidal, and the switching is timed to occur at the time when the current or voltage reaches zero. Parasitic capacitances and leakage inductances, which are normally considered a problem in such circuits operating at high frequencies, are incorporated in the circuit to define the resonance characteristics of the converter. An example of a quasi-resonant converter circuit can be found in U.S. Pat. No. 4,415,959 (Vinciarelli), which describes the cycles of charging a resonant capacitor during the on part of the operation cycle, and then, when the charging current reaches zero, switching the switch off, whereupon the output inductor discharges the resonant capacitor, transferring the energy to the load. By switching at zero current, this topology reduces switching losses, which allows the converter to run at a higher frequency.
However, such quasi-resonant converter circuits still require relatively large capacitors for storing the amounts of charge necessary, and at the operating voltage. Switching timing is also critical, which means that the control of the circuit operation is a non-trivial task, and may require a relatively complex design, or very low tolerance (ie expensive) resonance components and care over modelling parasitic. If the circuit is switched sub-optimally, or not correctly matched to the resonant components, then switching losses and inefficiency can result.
The invention also relates to arrangements of two or more convertors operated in parallel. As is already known, there are significant advantages in using two or more converter circuits to transfer energy in parallel from a common power source to a common load: U.S. Pat. No. 5,796,595 (Cross) describes a converter circuit which includes two soft-switching resonant converters operating in parallel. The switching sequence is interleaved, phase shifted by 180° between the two circuits, so as to provide power transfer during both halves of the switching cycle. Switching is carefully timed such that the switches are turned on while the voltage across them, and the current through them, are near zero, thus reducing switching losses. This converter circuit, however, still requires a relatively large input capacitor to cope with the voltage swings across it and a relatively complicated control unit to generate the required pulse-width modulation and interleaved soft-switching timing in the primary and secondary switches. Since it is based around the use of transformers with leakage inductance, it also uses a clamp circuit to recycle energy stored in the leakage inductance after each cycle in order to achieve zero-voltage switching of the power switches.
U.S. Pat. No. 5,563,780 (Goad) describes a power converter in which multiple smaller converters are connected in parallel between the power source and the load, and in which the smaller converters are switched sequentially and pulse-width modulated such that at least two of converters are always on at any one time.
It is an aim of the invention to provide a high-frequency DC-DC converter which provides continuous energy transfer at very high efficiency, with very low ripple voltage on the output, which requires less smoothing on the output, which has significantly reduced switching losses, which has simplified control circuitry, which does not require balancing circuitry, which requires a small number of components and in which the power rating of the components can be reduced relative to the overall power rating of the converter.